Various types of digital hearing aids have been proposed. The audio input, sensed by a microphone, is digitized, for example in an analog/digital (A/D) converter. The digitized signals or data are then applied to a microprocessor, for processing therein in accordance with a predetermined transfer function. The transfer function is stored in a memory, for example a read-only memory (ROM). After conversion in accordance with the transfer function as determined by the structure, and programming of the ROM, the digitized signal is reconverted in a D/A converter to an analog signal, for application to an electro-acoustic transducer, such as an earphone or an earplug.
It is known that the hearing acuity, with respect to frequency and amplitude of hard-of-hearing persons changes with time. It is, therefore, necessary to test hearing of the person and, if necessary, to exchange the hearing aid for one which is matched to the particular correction requirements of the user.